1. Field of the Invention
The present invention relates to a development apparatus of an electrophotographic image forming apparatus of an electrophotographic type, such as copying machine, laser beam printer, facsimile and complex machine (hereafter referred simply to as an “image forming apparatus”).
2. Related Background Art
In an image forming apparatus of an electrophotographic type or electrostatic recording type, a latent image is formed on a electrostatic latent image bearing member, for example, a photosensitive drum, and a visible image is obtained by attaching a developer (toner) to this latent image. Particularly, in a color image forming apparatus for forming chromatic color images, a development method using a two-component developer including a toner and a carrier is used widely since stability of image quality and durability of the apparatus are superior to those of other development methods.
In the development method using a two-component developer, including a nonmagnetic one component toner and a magnetic carrier (hereafter referred simply to as “toner and carrier”) being charged in the development apparatus, toner alone is being consumed for development of electrostatic latent image. Therefore, toners should be replenished freshly one-by-one to the development apparatus for each of the colors. In order to maintain an electrostatic latent image at a predetermined development concentration at all times, an amount of the toner to be replenished should be controlled strictly. For example, when the toner is replenished from toner cartridges corresponding to each of Y, M, C and K colors to the development apparatus, generally, those with such a structure that the toner is fed in a pushing manner by rotating a powder conveyance screw having a spiral structure which is provided for every color, and the conveyance amount is controlled, have been frequently used. Reasons why the powder conveyance screw is frequently used are that the conveyance amount per one rotation of the screw can be determined easily with a simple structure thereof and that necessary controls can be accomplished with reduced costs.
In recent years, it has been requested that images with various image ratios, from large images having a large image area, such as photographs, to small images having a small image area, such as one-point-color, should be output at high speed and with stable manner. In this case, use of above-mentioned powder conveyance screw for toner replenishment (hereafter referred to as “toner conveyance screw”) involves the following problems:
As for the toner conveyance screw, a screw shaft equipped with a blade in a spiral form penetrates through a screw pipe and is rotated to feed the toner in a pushing manner by the spiral blade in the pipe. A minimal allowance clearance is provided between an outside diameter of the spiral blade and an inside diameter of the screw pipe so as to enable rotation of the spiral blade. In some cases, a so-called “flashing phenomenon” in which the toner leaks out from such clearance and is supplied more than necessary to the development apparatus, is generated, thereby posing a problem. In order to solve this flashing phenomenon problem, a toner replenishment apparatus which regulates a toner amount by adjusting the clearance is proposed (see, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 5-224530
However, even when a toner amount is regulated by adjusting the clearance as is the case of the toner replenishment apparatus disclosed in the patent publication of above-mentioned Patent Document 1, there still remain unresolved problems.
In recent years, from the viewpoints of higher image quality, energy saving and speeding up of copying operation, there has been a tendency towards use of a toner having smaller diameter particles and a lower melting point. Therefore, toners with a higher degree of cohesive force (or sticking power) which is one of factors for determining the powder fluidity, namely, higher cohesion degree toners, have been frequently used. Therefore, when runout or eccentricity is caused to the toner conveyance screw while rotating, toner is ground in the clearance with regard to the screw pipe, thereby generating toner cohesion clusters. These toner cohesion clusters result in defective images, such as a void image or a stain on the image. Particularly, with copying machines in which a toner conveyance screw is used frequently to replenish toner from a toner cartridge to a development apparatus, suppression of generation of cohesion cluster as mentioned poses a significant problem.
In addition, as a known development method in color image forming apparatus, a rotary-type development unit is mentioned. For example, this method has such a configuration that a plurality of development apparatuses corresponding to each of Y, M, C, K are equally distributed on the same circumference in a radial pattern and are displaced in a rotational manner and are rotated to a position facing an electrophotographic photosensitive drum (hereafter referred simply to as a “photosensitive drum”) that is a latent image bearing member to initiate development. In this case, for example, cartridges to which each color of Y, M, C, K toner are charged are arranged in one row in a tandem manner and are provided at upper portion of a rotary-type development unit to increase the amount of toner accommodation as much as possible.
In this case, each of the development apparatuses corresponding to Y, M, C, K arranged in a radial manner in the rotary-type development unit which is a rotating body is connected through a toner replenishment path to each of those corresponding to a plurality of cartridges arranged in one row at an upper portion thereof. Therefore, it is natural from geometrical viewpoints that there is a dimensional difference between each length of the replenishment path corresponding to Y, M, C, and K. Thus, a toner conveyance screw is arranged to each of toner replenishment paths having dimensional differences to form a part of the replenishment path, and therefore, a length of the screw shaft and a length of the screw pipe are also different for Y, M, C, and K. If the length of toner conveyance screw is different for Y, M, C, K, there is also a difference of the time for the toner to pass through the screw pipe resulting in a difference of generation of toner cohesion clusters. In other words, replenishing the toner of the same component uniformly from the toner cartridge to each of development apparatuses does not constitute a fundamental solution for suppression of the generation of toner cohesion clusters and for prevention of defective images due to a void image or a stain on the image.